Male mice reared in isolation from weaning until sexual maturity exhibit aggressive behavior towards other intact males, as well as towards bulbectomized test males which are noncombative. Encounters with intact males that are behaviorally interactive provide a finer scale for discriminating degrees and styles of aggressive behavior than encounters with less interactive test animals. Our previous research has demonstrated that sensitive periods for preweaning handling variables are strain specific, and some appear to be associated with the X-chromosome. The DBA/1Bg Y-chromosome is associated with high aggression scores in the presence of components of its own autosomal complement, as well as with a supranormal pubertal testosterone surge. Various factors affecting growth rate and maturation are also correlated with variations in aggression and are being investigated in relation to effects on testis and seminal vesicle weights and morphology, as well as plasma testosterone levels in order to determine whether this is the primary mechanism by means of which they affect aggression. We have demonstrated that multiple sound induced seizures administered at weaning significantly reduce fighting behavior at sexual maturity, and that this effect is possibly central and does not involve male hormone. We are now successfully banding the Y-chromosome and will continue our investigations on the possible distinguishing characteristics of the DBA/1Bg Y (associated with high aggression) on the hypothesis that these may also occur in other mammals in association with the same behavioral propensities. The possibility that the H-Y antigen may be different for this chromosome is also being studied. An outbred stock containing a pigment mutation and showing spontaneous aggression among both males and females is being investigated. In addition to providing a tool for the study of factors affecting female aggression, our findings suggest that genes linked to particular coat color markers are involved in the predisposition to fighting and could be used to identify one or more autosomes that appear to make contributions to aggressive behavior.